Dehydrated vegetables are used widely in products such as soups and other processed foods, often in dehydrated, diced form. The major problem associated with dehydrated vegetables is that, on rehydration, they often remain collapsed and shrunk, giving rise to a product of poor texture relative to the textural quality of the fresh or minimally processed vegetables. The irreversible shrinkage that occurs during drying is due to the collapse of the cell-wall structure.
Pectin is a major component of vegetable cell walls and, as such, makes a major contribution to the structural integrity and strength of vegetables and vegetable pieces. Native pectin is, generally, high methoxyl in structure and, as a result, is susceptible to degradation on heating under neutral pH conditions. This form of degradation occurs during cooking and is the major reason for unwanted textural softening of vegetables on heat processing. During forced-air drying of vegetable pieces, water is removed, thus releasing the turgor pressure and causing the cell wall structure to collapse. The collapse can be irreversible, if the moisture content of the materials is very high or if the cell wall structure is weak. In these cases, rehydration only causes the absorption of water into the intercellular spaces, thus leading to poor rehydratability of the dehydrated material and poor texture on rehydration.
Vegetables contain pectinmethylesterase (PME) in their cell walls. This endogenous enzyme can be activated by mild heat treatment under specific conditions of pH and ionic strength and, when activated, leads to demethoxylation of the native pectin. The resulting demethoxylated pectin has a structure that is more stable against degradation on heating at neutral pH and that more effectively cross-links in the presence of calcium ions. Both of these factors lead to the strengthening of the cell wall structure and pretreatments that activate the PME enzyme in vegetables have been shown to give improved texture after freezing and canning.
EP-A-0337621 discloses a method for producing dried fruit which involves stabilizing the color and flavor of the fruit by infusing it in a 10 to 70% carbohydrate solution prior to drying.
EP-A-0404543 also describes a process for preparing dried fruit which includes infusion in an aqueous carbohydrate solution. This document teaches that the fruit must be soaked under conditions which deactivate the enzymes and also soften the fruit prior to dehydration.
Our related application, EP-A-0663152, published on Jul. 19, 1995, is based on the finding that improved textural qualities on rehydration may be obtained for dehydrated vegetables by preheating the vegetables to activate endogenous PME before blanching and drying. This effect is believed to be a result of the preservation of the cell wall structure due to the demethoxylation of pectin.
It has now been found that an improved dehydrated vegetable product can be obtained if the above-mentioned pretreatment to activate PME is used in conjunction with microwave-vacuum drying techniques.
Therefore, an object of the present invention is to provide an improved process for dehydrating vegetable matter.
Another object of the present invention is to provide dehydrated vegetable matter having good rehydratability.
A further object of the present invention is to provide dehydrated vegetable matter having good texture and flavor upon rehydration.